1. Field of the Invention
The present invention relates to an optical system having a long back focus, a compact structure, and good optical characteristics and, for example, to an optical system suitable for a projection lens of a high-definition mobile liquid crystal projector.
2. Related Background Art
In recent years, the liquid crystal projectors have commonly been utilized as devices capable of presenting a projection of an image of a personal computer or the like on a large screen in conferences, presentations, and so on. Among others, it is preferable to use a three-panel-type liquid crystal projector using three liquid crystal displays (LCDs) for the respective colors of R (red), G (green), and B (blue) and arranged to project a synthetic image of image information displayed on the respective LCDs, onto the screen, in order to present a high-definition image with high contrast. Projection lenses of such three-panel-type liquid crystal projectors are desirably those satisfying the following requirements:
1) The back focus of a certain length is ensured in order to provide a space for placement of such elements as a dichroic prism and polarizers for combining color beams from the liquid crystal displays for the respective colors of R, G, and B, between the liquid crystal displays and the projection lens;
2) The projection lens is an optical system telecentric on the liquid crystal display side whose pupil viewed from the liquid crystal display (reduction) side is at infinity, in order to minimize influence of angle dependence of color combining films and in order to ensure good pupil matching with an illumination system;
3) Corresponding pixels of the respective color liquid crystal displays have to be well superimposed throughout the entire screen in order to prevent degradation of resolution sense and quality due to doubling of characters etc. of the personal computer when the synthetic image of the image information of the three color liquid crystal displays is projected onto the screen.
For that purpose, chromatic deviation (chromatic aberration of magnification) occurring in the projection lens has to be corrected well in the visible band (to realize an apochromat system);
4) The optical system is corrected so that the maximum distortion is not more than about 2% in order to prevent the projected image from being distorted in the contour part and degrading (particularly, if there remains sudden distortion change or the like in the periphery and intermediate part or the like the quality of image will degrade and thus it is not preferable); and so on.
The prior arts intended for meeting the above requirements include, for example, the projection lenses for liquid crystal projectors as described in Japanese Patent Applications Laid-Open No. H10-186235 and No. H10-268193. The projection lenses disclosed in these applications are comprised of five lens units having their respective optical powers which are negative, positive, positive, negative, and positive in the order named from the enlargement conjugate side, and magnification variation (zooming) from the wideangle extreme to the telephoto extreme is implemented by fixing the first and fifth lens units and moving all the second to fourth lens units in the lens system toward the enlargement conjugate side. Since the first and fifth lens units closest to the enlargement conjugate side and to the reduction conjugate side are fixed, the total length is maintained constant during zooming. This optical system is also a telecentric zoom lens having the back focus of a fixed length and realizing low distortion and low chromatic aberration.
There are, however, strong market needs for achievement of much smaller mobile liquid crystal projectors at present and toward the future.
For this theme of xe2x80x9cdownsizing of liquid crystal projector,xe2x80x9d the size of the liquid crystal displays providing the original images first has to be decreased. However, for ensuring the resolution of the same level as before, the aperture ratio of the liquid crystal displays is reduced and the ratio of sizes of illuminated areas and emission light sources (=size of illuminated areas/size of light sources) becomes smaller, so that illumination efficiency is usually lowered. This will results in losing commercial values in respect of brightness even though the apparatus can be downsized.
Against the above problem, in order to ensure screen illuminance enough for use even under fluorescent tubes and the daylight, it was difficult to attain sufficient brightness with marketability by the full-aperture F-value at the wideangle extreme of about 2.5 (the F-values hereinafter representing full-aperture values) in spite of the projection lens for compact projector. It is thus necessary to attain brightness enough to maintain marketability, by setting the F-value to approximately 2.0.
However, the decrease of the F-value to about 2.0 or to below 2.0 will raise the possibility of forcing constraints from the F-value on design, decreasing freedom of correction of aberration, making correction of aberration, particularly, spherical aberration very hard, and thus degrading the optical characteristics. It is possible to well correct the spherical aberration by use of many lenses, but it will result in increasing the scale of the optical system and thus lowering the commercial values as a mobile liquid crystal projector.
The cross dichroic prism mainly used in the liquid crystal projectors and disclosed in Japanese Patent Application Laid-Open No. H11-95098 is very compact, but it is very difficult to fabricate the prism and join its prism elements. In order to decrease the production cost of the color combining prism, a method in which the dichroic mirror layers do not intersect with each other inside the prism, is also investigated as a new color combining method.
An object of the present invention is to provide an optical system having good optical characteristics while realizing a large aperture ratio. A further object of the present invention is to provide an optical system that can be constructed in a total length of very small value in combination with a cross dichroic prism and that can be constructed without increasing the scale of the entire system and realized in the scale approximately equal to that of the conventional systems using the cross dichroic prism, even in combination with a prism of a color combining method with a longer prism path length, other than the cross dichroic prism.
An optical system according to the first aspect of the present invention is an optical system comprising a first optical component, a second optical component, a third optical component, a fourth optical component, a fifth optical component, and a sixth optical component in the order named from the enlargement conjugate side,
wherein at least four optical components out of said plurality of optical components move during zooming and said first optical component closest to the enlargement conjugate side has a negative optical power.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein during zooming the optical component closest to the enlargement conjugate side and the optical component closest to the reduction conjugate side both are fixed relative to a reduction-side conjugate point.
An optical system according to another aspect of the invention is the optical system according to the first aspect, which comprises at least one optical element having an aspherical surface.
An optical system according to another aspect of the invention is the optical system according to the first aspect, which comprises at least one optical element comprised of a material with an Abbe""s number of not less than 80.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein during zooming at least two movable optical components integrally move.
An optical system according to another aspect of the invention is the optical system according to the above aspect wherein said second optical component and said fifth optical component integrally move during zooming.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said second optical component has a positive optical power, said third optical component a positive optical power, said fourth optical component a negative optical power, and said sixth optical component a positive optical power.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein during zooming said second to fifth optical components move.
An optical system according to another aspect of the invention is the optical system according to the above aspect wherein during zooming from a wideangle extreme to a telephoto extreme the second to fifth optical components all move toward the enlargement conjugate side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
0.5 less than xe2x88x92xcex223w less than 1.0xe2x80x83xe2x80x83(1),
where xcex223w is a magnification at a wideangle extreme in combination of the second optical component and the third optical component.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
1.0 less than xcex24w less than 5.0xe2x80x83xe2x80x83(2),
where xcex24w is a magnification at a wideangle extreme of the fourth optical component.
An optical system according to another aspect of the invention is the optical system according to the first aspect, which comprises a stop, wherein said stop is disposed in the second or third optical component.
An optical system according to another aspect of the invention is the optical system according to the above aspect wherein during zooming said stop moves integrally with said third optical component.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein at least one optical component out of the first, fifth, and sixth optical components includes an optical element having an aspherical surface.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the fifth optical component comprises at least one biconcave lens and at least two positive lenses in the order named from the enlargement side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
1.2 less than f6/fw less than 2.2xe2x80x83xe2x80x83(3),
where fw is a focal length of the entire system at a wideangle extreme and f6 is a focal length of the sixth optical component.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein focus adjustment is implemented by moving the first optical component on the optical axis.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said third optical component consists of one positive lens.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said second optical component comprises a positive lens and a negative lens having a stronger power on the enlargement conjugate side than on the reduction conjugate side, in the order named from the enlargement conjugate side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said first optical component comprises a biconvex lens, a negative meniscus lens being convex on the enlargement conjugate side, a negative lens, and a positive lens having a stronger power on the enlargement conjugate side than on the reduction conjugate side, in the order named from the enlargement conjugate side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said first optical component comprises a biconvex lens, a negative meniscus lens being convex on the enlargement conjugate side, and a negative lens having a strong power on the enlargement conjugate side, in the order named from the enlargement conjugate side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
0.02 less than fw/f2 less than 0.6xe2x80x83xe2x80x83(4),
where fw is a focal length at a wideangle extreme of the entire optical system and f2 a focal length of said second optical component.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
xe2x88x921 less than fw/f1 less than xe2x88x920.3xe2x80x83xe2x80x83(5),
where f1 is a focal length of said first optical component and fw a focal length at a wideangle extreme of the entire optical system.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
0.45 less than fw/f3 less than 1.3xe2x80x83xe2x80x83(6),
where f3 is a focal length of said third optical component and fw a focal length at a wideangle extreme of the entire optical system.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
xe2x88x920.9 less than fw/f4 less than xe2x88x920.1xe2x80x83xe2x80x83(7),
where f4 is a focal length of said fourth optical component and fw a focal length at a wideangle extreme of the entire optical system.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
xe2x88x920.15 less than fw/f5 less than 0.35xe2x80x83xe2x80x83(8),
where f5 is a focal length of said fifth optical component and fw a focal length at a wideangle extreme of the entire optical system.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
0.2 less than fw/f6 less than 0.8xe2x80x83xe2x80x83(9),
where f6 is a focal length of said sixth optical component and fw a focal length at a wideangle extreme of the entire optical system.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said fourth optical component comprises one negative lens having a stronger power on the reduction conjugate side than on the enlargement conjugate side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said fifth optical component comprises a negative lens having a stronger power on the enlargement conjugate side than on the reduction conjugate side, a positive lens having a strong power on the reduction conjugate side, and a positive lens having a strong power on the reduction conjugate side in the order named from the enlargement conjugate side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein said sixth optical component comprises one positive lens having a stronger power on the enlargement conjugate side than on the reduction conjugate side.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein focus adjustment is implemented by moving said first optical component along the optical axis.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
0.8 less than bf/fwxe2x80x83xe2x80x83(10),
where bf is an air-reduced length between a reduction-side conjugate point and a surface closest to the reduction conjugate side in the optical component closest to the reduction conjugate side out of said at least six optical components when an enlargement-side conjugate point is at infinity from said optical system, and fw a focal length at a wideangle extreme of the entire optical system.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following conditions are satisfied:
0.2 less than M2/M3 less than 1xe2x80x83xe2x80x83(11),
0.4 less than M4/M3 less than 1xe2x80x83xe2x80x83(12),
0.4 less than M5/M3 less than 1xe2x80x83xe2x80x83(13),
where M2 is a moving distance of the second optical component during zooming from a wideangle extreme to a telephoto extreme, M3 a moving distance of the third optical component during the zooming from the wideangle extreme to the telephoto extreme, M4 a moving distance of the fourth optical component during the zooming from the wideangle extreme to the telephoto extreme, and M5 a moving distance of the fifth optical component during the zooming from the wideangle extreme to the telephoto extreme.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein during zooming from a wideangle extreme to a telephoto extreme, a spacing between the first optical component and second optical component decreases, a spacing between the second optical component and third optical component decreases, a spacing between the third optical component and fourth optical component increases, and a spacing between the fifth optical component and sixth optical component increases.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein the following condition is satisfied:
|fw/tkw| less than 0.25xe2x80x83xe2x80x83(14),
where fw is a focal length at a wideangle extreme of the entire optical system and tkw a distance between a reduction-side conjugate point at the wideangle extreme and a position of a reduction-side pupil.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein a full-aperture F-value at a wideangle extreme of the optical system is not more than 2.3.
An optical system according to another aspect of the invention is the optical system according to the first aspect wherein a full-aperture F-value at a wideangle extreme of the optical system is not more than 2.0.
A projection optical system according to the second aspect of the present invention is a projection optical system for projecting light from a plurality of image forming elements onto a projection surface, comprising color combining means for combining light beams of different colors from the plurality of image forming elements,
wherein the following condition is satisfied:
1.2 less than bf/ppath less than 1.5xe2x80x83xe2x80x83(16),
where bf is an air-reduced length between the image forming elements and the projection optical system when an enlargement-side conjugate point is at infinity from said projection optical system, and ppath an air-reduced length of said color combining means.
A projection optical system according to the third aspect of the invention is the projection optical system according to the second aspect wherein said color combining means comprises three or more prisms and has two dichroic mirror layers for reflecting light beams of mutually different wavelength regions, and wherein the two dichroic mirror layers do not intersect with each other inside said color combining means.
A projection optical system according to another aspect of the invention is the projection optical system according to the third aspect wherein said color combining means comprises a prism having a surface serving as a total reflection surface and as a transmissive surface, at a position closest to the exit side, and wherein, when a prism disposed between said two dichroic mirror layers is cut by a plane along a direction of color composition, an outline of a cross section thereof is constructed of four or more lines and at least one of bending points in a cross section of surfaces on which the dichroic mirror layers are not formed, exists inside of the prism with respect to a line connecting two ends of lines in a cross section of surfaces on which the two dichroic mirror layers are formed.
A projection optical system according to another aspect of the invention is the projection optical system according to the third aspect wherein said color combining means comprises a prism having a surface serving as a total reflection surface and as a transmissive surface, at a position closest to the exit side, and wherein, when a prism located between said two dichroic mirror layers is cut by a plane along a direction of color composition, an outline of a cross section thereof is constructed of four or more lines and at least one interior angle in the outline of the cross section is an angle exceeding 180xc2x0.
A projection optical system according to another aspect of the invention is the projection optical system according to the third aspect wherein said color combining means comprises four or more prisms, and comprises a prism having a surface serving as a total reflection surface and as a transmissive surface, at a position closest to the exit side, wherein the color combining means further comprises two dichroic mirror layers for reflecting light beams of mutually different wavelength regions, and wherein a plurality of prisms are disposed between the two dichroic mirror layers.
A projection optical system according to another aspect of the invention is the projection optical system according to the third aspect wherein said color combining means comprises a first prism having three or more optically smooth surfaces, a second prism having two or more optically smooth surfaces, a third prism having three or more optically smooth surfaces, and a fourth prism having two or more optically smooth surfaces, in the order named from the exit side of light, wherein said color combining means comprises a dichroic mirror layer for reflecting a first color beam on either one or both of opposed surfaces of the first prism and the second prism, and wherein said color combining means comprises a dichroic mirror layer for reflecting a second color beam on either one or both of opposed surfaces of the third prism and the fourth prism.
A projection optical system according to another aspect of the invention is the projection optical system according to the third aspect wherein said color combining means comprises a first prism having three or more optically smooth surfaces, one of said three or more surfaces being a surface serving as a transmissive surface and as a total reflection surface, a second prism having two or more optically smooth surfaces, a third prism having three or more optically smooth surfaces, and a fourth prism having two or more optically smooth surfaces, in the order named from the exit side of light, wherein said color combining means comprises a dichroic mirror layer for reflecting a first color beam on either one or both of opposed surfaces of the first prism and the second prism, and wherein said color combining means comprises a dichroic mirror layer for reflecting a second color beam on either one or both of opposed surfaces of the third prism and the fourth prism.
A projection optical system according to another aspect of the invention is the projection optical system according to the second aspect, comprising at least six optical components having respective optical powers,
wherein at least four optical components out of said at least six optical components move during zooming and the first optical component closest to the enlargement conjugate side has a negative optical power.
A projection optical system according to the fourth aspect of the invention is a projection optical system comprising:
color combining means for combining light beams of different colors from a plurality of image forming elements, and
the optical system according to the first aspect of the invention, as a projection optical system for projecting the light beams from said plurality of image forming elements, onto a projection surface,
wherein the following condition is satisfied:
1.2 less than bf/ppath less than 2.1xe2x80x83xe2x80x83(15),
where bf is an air-reduce length between the image forming elements and the projection optical system when an enlargement-side conjugate point is at infinity from said projection optical system, and ppath an air-reduced length of said color combining means.
A projection optical system according to another aspect of the invention is the projection optical system according to the fourth aspect wherein the following condition is satisfied:
1.2 less than bf/ppath less than 1.5xe2x80x83xe2x80x83(16).
A projection optical system according to the fifth aspect of the invention is the projection optical system according to the fourth aspect wherein said color combining means comprises three or more prisms and has two dichroic mirror layers for reflecting light beams of mutually different wavelength regions, and wherein the two dichroic mirror layers do not intersect with each other inside said color combining means.
A projection optical system according to another aspect of the invention is the projection optical system according to the fifth aspect wherein said color combining means comprises a prism having a surface serving as a total reflection surface and as a transmissive surface, at a position closest to the exit side, and wherein, when a prism disposed between said two dichroic mirror layers is cut by a plane along a direction of color composition, an outline of a cross section thereof is constructed of four or more lines and at least one of bending points in a cross section of surfaces on which the dichroic mirror layers are not formed, exists inside of the prism with respect to a line connecting two ends of lines in a cross section of surfaces on which the two dichroic mirror layers are formed.
A projection optical system according to another aspect of the invention is the projection optical system according to the fifth aspect wherein said color combining means comprises a prism having a surface serving as a total reflection surface and as a transmissive surface, at a position closest to the exit side, and wherein, when a prism located between said two dichroic mirror layers is cut by a plane along a direction of color composition, an outline of a cross section thereof is constructed of four or more lines and at least one interior angle in the outline of the cross section is an angle exceeding 180xc2x0.
A projection optical system according to another aspect of the invention is the projection optical system according to the fifth aspect wherein said color combining means comprises four or more prisms, and comprises a prism having a surface serving as a total reflection surface and as a transmissive surface, at a position closest to the exit side, wherein the color combining means further comprises two dichroic mirror layers for reflecting light beams of mutually different wavelength regions, and wherein a plurality of prisms are disposed between the two dichroic mirror layers.
A projection optical system according to another aspect of the invention is the projection optical system according to the fifth aspect wherein said color combining means comprises a first prism having three or more optically smooth surfaces, a second prism having two or more optically smooth surfaces, a third prism having three or more optically smooth surfaces, and a fourth prism having two or more optically smooth surfaces, in the order named from the exit side of light, wherein said color combining means comprises a dichroic mirror layer for reflecting a first color beam on either one or both of opposed surfaces of the first prism and the second prism, and wherein said color combining means comprises a dichroic mirror layer for reflecting a second color beam on either one or both of opposed surfaces of the third prism and the fourth prism.
A projection optical system according to another aspect of the invention is the projection optical system according to the fifth aspect wherein said color combining means comprises a first prism having three or more optically smooth surfaces, one of said three or more surfaces being a surface serving as a transmissive surface and as a total reflection surface, a second prism having two or more optically smooth surfaces, a third prism having three or more optically smooth surfaces, and a fourth prism having two or more optically smooth surfaces, in the order named from the exit side of light, wherein said color combining means comprises a dichroic mirror layer for reflecting a first color beam on either one or both of opposed surfaces of the first prism and the second prism, and wherein said color combining means comprises a dichroic mirror layer for reflecting a second color beam on either one or both of opposed surfaces of the third prism and the fourth prism.
An image projection apparatus according to another aspect of the invention is an image projection apparatus comprising:
an image forming element for forming an image, and
the optical system according to the first aspect of the invention,
wherein the image formed by said image forming element located at a reduction-side conjugate point of said optical system is enlarged and projected.
An image projection apparatus according to another aspect of the invention is the image projection apparatus according to the above aspect wherein said image forming element is a liquid crystal display device.
An image pickup apparatus according to another aspect of the invention is an image pickup apparatus comprising:
the optical system according to the first aspect of the invention, and
a photosensitive member disposed at a reduction-side conjugate point of said optical system,
wherein an image of an object is projected onto said photosensitive member disposed at the reduction-side conjugate point, using said optical system.
An image projection apparatus according to another aspect of the invention is an image projection apparatus comprising:
an image forming element for forming an image, and
the projection optical system according to the second aspect of the invention,
wherein the image formed by the image forming element disposed at a reduction-side conjugate point is enlarged and projected, using said projection optical system.
An image projection apparatus according to another aspect of the invention is the image projection apparatus according to the above aspect wherein said image forming element is a liquid crystal display device.
An image pickup apparatus according to another aspect of the invention is an image pickup apparatus comprising:
the projection optical system according to the second aspect of the invention, and
a photosensitive member disposed at a reduction-side conjugate point of said projection optical system,
wherein an image of an object is projected onto said photosensitive member disposed at the reduction-side conjugate point, using said projection optical system.